Disconnect your XP drive, connect the new drive, enter the BIOS and make the new drive the boot drive, and install vista on it.
After Vista is installed, connect the XP drive back.
Now, when you start the computer, press the function key that gives you the boot menu and select which drive to boot to. That way you can boot to either OS.
You can set the default drive by editing the BIOS settings. You won't have to do anything to boot to the default OS.
This keeps the two installations completely independent of each other. You can remove one or re-install one and nothing will change.
Alternatively, just leave the XP drive attached while you install Vista.
Then, you will not have to press any function keys when you start. Vita will give you a menu every time you start to choose the OS to use.
But, if something happens to the Vista drive one day, you will not be able to boot to XP until you restore the XP boot loader.